Phase noise reduction device

ABSTRACT

A device for reducing the phase noise of a signal (S in ), coming from a quasiperiodic source of fundamental frequency f 0 , comprises a superconducting circuit with an active line for voltage pulse transmission by transferring quanta of flux φ 0 . This circuit is defined so as to have a characteristic frequency f c  such that 0.3 f c , is less than or equal to the fundamental frequency f 0  of the quasiperiodic signal (S in ) applied as input, and delivers, as output, a voltage pulse signal of fundamental frequency f 0 .

The present invention relates to a device for reducing the phase noisein a signal coming from a quasiperiodic source.

It applies more particularly to superconducting logic circuits,especially to logic circuits in RSFQ (Rapid Single Flux Quantum)technology.

In general, logic systems use at least one clock signal for thesequencing and synchronization functions. The clock signals are usuallygenerated by oscillators. These quasiperiodic signals are not completelypure, despite the integration of resonant filters in the oscillators. Ifwe consider the representation of the spectral density of aquasiperiodic signal generated by an oscillator, a noise floor is thusobserved. This is the white noise of the spectrum, corresponding to ashort-term phase noise of the quasiperiodic signal. The phase lockcircuits normally used in digital systems (computers or other systems)do not allow this short-term phase noise to be reduced—their action hasa long-term stabilizing effect in order to prevent frequency drifts.

In what follows, the term “phase noise” is understood to mean the noisecorresponding to the noise floor or white noise of the frequencyspectrum of the signal. The subject of the invention is a device forreducing this phase noise. Such a device is particularly beneficial inthe field of rapid digital electronics. In particular, it makes itpossible to reduce the jitter in the clock signal, this beingparticularly irksome in digital circuits operating at high and very highfrequency.

In rapid digital electronic systems, a logic family usingsuperconducting circuits has been developed. This is the RSFQ (RapidSingle Flux Quantum) logic family based on the use of the quantizationof the magnetic flux and the transfer of single flux quanta φ₀. In thisapproach, the logic data processing amounts to manipulating voltagepulses resulting from the passage of the flux quanta in current loops.One of the basic elements of this logic family based on superconductorsis the shunted Josephson junction, which allows a single flux quantum tobe transferred or retained, the passage of a flux quantum into thejunction resulting in the appearance of a voltage pulse at its terminalssuch that ∫Vdt=h/2e=φ₀=2.07×10⁻¹⁵ weber (h being Planck's constant).With current technologies, the voltage pulse therefore has an amplitudeof the order of 2 millivolts over 1 picosecond.

Each junction is defined by a critical current I_(c) and a normalresistance R_(n), dependent on its geometry and on the technology used.The propagation/transfer function is provided by a bias current controlof the appropriate junction, which allows the current flowing throughthe junction to be increased or decreased, thus making it possible toretain the flux quantum in the loop or to transfer the flux quantumthrough the junction into the next loop.

RSFQ logic has resulted in many logic circuits such as analog/digitalconverters, random access memories and processors for signal processingthat calculate rapid Fourier transforms, which may operate at very highfrequency. The upper operating limit of RSFQ logic elements is given bytheir critical frequency, which depends on their geometry and on thetechnology employed (three-layer, planar, etc.). This characteristicfrequency is given by the following equation:f _(c) =I _(c) R _(n)/φ₀where I_(c), is the critical current of the junction, R_(n) is thenormal resistance and φ₀ is the flux quantum, equal to 2.07×10⁻⁵ weber.

A useful review of applications in RSFQ logic will be found in thearticle by Konstantin K. Likharev “Progress and prospects ofsuperconducting electronics”, Superconducting Science Technology, 3(1990), pages 325-337.

Another active element of RSFQ logic is the Josephson transmission line.A Josephson transmission line is a line comprising parallel-shuntedJosephson junctions coupled between them by superconducting inductors.Such a line allows propagation of single flux quanta, and thereforeserves as a logic data transport medium. A very short voltage pulse, ofthe order of 2 millivolts over 1 picosecond, applied as input of such aline, propagates along this line by propagation of a flux quantum φ₀,also called a fluxon, through permanent current loops. This voltagepulse is recovered at the output. These Josephson transmission linesallow logic pulse transmission without any distortion.

If two pulses are applied in succession as input, two fluxons aregenerated in the line and propagate along this line. These two fluxonsare separated in the line by a distance representative of the timeinterval separating the two pulses applied as input. However, because ofa repulsive interaction between the fluxons generated, if the distance dbetween the two fluxons is short enough for this repulsive interactionto be of significant strength, spatial redistribution takes place in theline, which is manifested at the output by a time interval separatingthe two pulses that differs from that observed at the input of the line.In other words, one pulse has been accelerated and the other slowed downin the line. This effect has been clearly explained in an articleentitled “Fluxon interaction in an overdamped Josephson transmissionline” by V. K. Kaplunenko, Applied Physics Letters, 66 (24), Jun. 12,1995, with a numerical illustration of this effect observedexperimentally on a Josephson transmission line comprising 200 shuntedJosephson junctions coupled in parallel by a superconducting inductorand having a characteristic frequency f_(c), of 104 GHz. Two voltagepulses 9.6 ps (picoseconds) apart, corresponding to f_(c) ⁻¹, areapplied as input to this line. The time interval between the two fluxonspropagating along the line increases. At the output, two voltage pulses27 ps apart are obtained. Owing to the repulsion between the fluxons,one pulse has been slowed down and the other speeded up, resulting in anincrease in the time interval separating the two pulses. Thismodification phenomenon is observed in practice only for an interfluxondistance corresponding to a time interval of less than a saturation timeof the junction, evaluated to 3 f_(c) ⁻¹, i.e. around 28.8 ps in theexample. If the distance between the fluxons is too great, the force isnot high enough. It is therefore necessary for the fluxons generated tobe sufficiently close so that the force is high. In the example, if twopulses 30 picoseconds apart are injected into the line, this timeinterval at the output of the line is unchanged.

A sequence of bits representing logic data may thus be modified in theJosephson transmission line owing to the effect of the repulsiveinteraction between the fluxons, this being equivalent to a loss oflogic information. In a logic system, this loss of information may haveserious repercussions, namely raw information loss, desynchronization(phase comparator), etc. To avoid this interaction problem, the authorof the article recommends designing the line so that the time intervalbetween two fluxons generated in the line is not less than 3 f_(c) ⁻¹,i.e. 28.8 ps (saturation value) in the example. A suitable design isobtained in particular by varying the critical current, the normalresistance and the inductances in the definition of the circuit. Theinteraction effects can then be reduced in operation by varying the biascurrent of the Josephson junctions.

In the invention, this repulsive interaction effect between the fluxonsis of use for withdrawing an advantageous technical effect therefrom, inrespect of the filtration of the white noise of a signal coming from aquasiperiodic source. The basic notion of the invention is to use thiseffect on a series of pulses from a clock signal coming from anyquasiperiodic source of fundamental frequency f₀ in order to lower thewhite noise level of this signal relative to the level of thefundamental. This is because, if we take the case of a clock signal ofthe type consisting of voltage pulses, the white noise level results ina temporal dispersion of the pulses of the signal, and consequently in adispersion of the spatial distance between the fluxons generated in thesuperconducting transmission line.

The interaction effect over the entire length of the line means that aredistribution of the fluxons within the confined space of the line isobserved, due to the random behavior of large numbers about a smoothvalue, corresponding to a mean value of the interfluxon distance. Thisspatial redistribution of the fluxons has as direct effect the temporalredistribution of the pulses at the output.

The white noise of the quasiperiodic signal is manifested, on thesignal, by a temporal dispersion of the pulses and, in thesuperconducting transmission line, by a dispersion of the spatialdistance between two successive fluxons.

Owing to the periodic nature of the signal at the input, the fluxons areorganized in the line as a periodic lattice. In the Josephsontransmission line, this is a one-dimensional periodic lattice along thedirection of propagation of the flux quanta. After a certain number ofpulses, corresponding to a transient delay, a redistribution of thislattice takes place, with a smooth interfluxon distance around a meanvalue. Thus the phenomenon of interfluxon repulsion, combined with thestatistics of large numbers, leads to a uniform redistribution of thefluxons within the lattice, thereby resulting, at the output of theline, in a reduction in the white noise level of the quasiperiodicsignal.

In general, according to the invention, taking any physical systemcapable of generating particles having repulsive interactions betweenthem for an inter-particle distance shorter than a saturation value ofthe system (characteristic frequency), such as electrons (quantroniccircuits), flux quanta or vortices, it is possible to reduce the phasenoise by reorganizing the particle lattice in the physical system.

The invention therefore relates to a device for reducing the phase noiseof a signal coming from a quasiperiodic source of fundamental frequencyf₀. According to the invention, this device comprises a physical systemfor transmitting pulses by transferring particles, said system beingdefined so as to have a characteristic frequency f_(c) defining anoperating frequency range of the device with a low limit that isdependent on said characteristic frequency, in such a way that, for thequasiperiodic signal applied as input, said particles have a mutualrepulsive interaction and said system delivering, as output, pulses atthe fundamental frequency f₀.

The invention also relates to a device for reducing the phase noise of asignal coming from a quasiperiodic source of fundamental frequency f₀.According to the invention, it comprises a superconducting circuit withan active line for voltage pulse transmission by transferring quanta offlux φ₀, said circuit being defined so as to have a characteristicfrequency f_(c) such that 0.3f_(c)≦f₀ where f₀ is the fundamentalfrequency of the quasiperiodic signal (S_(in)) applied as input, anddelivering, as output, a voltage pulse signal of fundamental frequencyf₀.

The phase noise reduction may be improved by defining a superconductingcircuit consisting of an active voltage pulse transmission line, suchthat the flux quanta generated in the circuit owing to the effect ofapplying the quasiperiodic signal are organized along a two-dimensionalperiodic lattice. Thus, the interactions between the flux quanta takeplace between closest neighbors along the two dimensions of the lattice.

The invention applies not only to the flux quanta generated in aJosephson transmission line, but more generally to any superconductingcircuit based on active voltage pulse transmission line. In particular,it also applies to vortex flux transmission lines, namely transmissionlines with a long Josephson junction, with Josephson vortex flux flow,with a slot or microbridge line, or with Abrikosov vortex flux flow.

The phase reduction device may furthermore be used advantageously in afrequency multiplier circuit.

Other advantages and features of the invention will become more clearlyapparent on reading the following description, given by way ofnon-limiting indication of the invention and with reference to theappended drawings in which:

FIG. 1, already described, illustrates the spectral density A(S_(in)) ofa signal coming from a quasiperiodic source;

FIG. 2 shows a circuit diagram of a phase reduction device according tothe invention based on a Josephson transmission line comprising aplurality of Josephson junctions;

FIG. 3 shows a first example of an embodiment of such a line, in abicrystal multijunction technology;

FIG. 4 a shows schematically a periodic lattice of fluxons generated bya pulse clock signal in the Josephson transmission line;

FIGS. 4 b and 4 c illustrate the phenomenon of temporal redistributionof the voltage pulses in such a line;

FIG. 5 a shows another example of an embodiment of a phase reductiondevice comprising two Josephson transmission lines placed in parallel inthe same surface plane;

FIG. 5 b is an illustration of the periodic lattice of the correspondingfluxons;

FIGS. 6 a and 6 b illustrate schematically two alternative ways of usingtwo Josephson transmission lines in parallel in a phase reduction deviceso as to improve the effectiveness of the correction;

FIG. 6 c is an alternative to the previous figures with n=3 Josephsontransmission lines in parallel, with an illustration of the periodiclattice of the corresponding fluxons;

FIG. 7 shows an example of the use of a phase noise reduction device ina frequency doubling circuit;

FIGS. 8 a and 8 b show another example of a phase reduction device basedon a Josephson transmission line produced in a ramp-edge junctiontechnology;

FIGS. 9 a and 9 b show two embodiments of a phase noise reduction devicebased on a long Josephson junction transmission line;

FIGS. 10 a and 10 b show a phase noise reduction device based on avortex-flux, slot or microbridge line; and

FIG. 11 is an illustration of the periodic lattice of the vorticesgenerated in such a line.

FIG. 1 shows the spectral density A(S_(in)) of a signal S_(in) comingfrom a quasiperiodic source and applied as clock signal in a logicsystem. In the invention, the aim is to reduce the phase noise/signalratio N₂/N₁, which is around −115 to −120 dB_(c) for signals coming fromconventional quasiperiodic sources (oscillators) by at least a factor of10. Such a reduction is particularly advantageous in the field ofelectronics operating at very high frequency and in particular insystems based on high-T_(c) (high critical temperature) superconductingRSFQ logic circuits in which the thermal noise is low. The benefit of asignal whose short-term noise has been singularly reduced is then put tofull use.

FIG. 2 illustrates a first embodiment of a phase noise reduction deviceaccording to the invention, comprising a superconducting circuit basedon a voltage pulse transmission line, at the input of which the signalS_(in) to be processed is applied, and the circuit delivers, as output,a signal S_(out) whose phase noise has been reduced.

In this example, the transmission line is a Josephson transmission linecomprising a plurality of Josephson junctions JJ₁, JJ₂, . . . JJ₂₀₀,shown as their simplified circuit diagram. The Josephson junctions areshunted, mounted in parallel, and coupled to one another viasuperconducting inductors Ls₁, Ls₂, Ls₃, . . . Ls₂₀₀. A superconductinginductor Ls₀ is also provided at the input, between an input signalelectrode A and the first Josephson junction JJ₁.

The input signal is applied to the terminals of the line, between twoinput signal electrodes A and M. The output signal S_(out) is obtainedat the output of the line, between two output signal electrodes B andM′. The electrodes M and M′ are the ground electrodes of the line. Thejunctions are biased with current I_(b), which is less than the criticalcurrent I_(c) of the junctions, so that a permanent current loop B_(c)is established in each cell closed off by a junction.

The application of a pulse at the input of such a line increases thecurrent of the junction to above the critical current. The Josephsoneffect occurs—a flux quantum passes through the current loop and acorresponding voltage pulse appears at the terminals of the junction.The voltage pulse thus propagates along the line, without beingdistorted.

If a clock signal pulse train is applied, a corresponding train isrecovered at the output. According to the invention, the characteristicsof the line are chosen so as to obtain a given characteristic frequencyf_(c). This characteristic frequency f_(c) defines an operatingfrequency range of the device with a low limit that depends on thischaracteristic frequency. For a quasiperiodic signal applied at theinput, the fundamental frequency of which lies within the operatingrange thus defined, effective repulsive interaction is obtained, therebymaking it possible to reduce the white noise background of this signal.

More particularly, the characteristics of the line are chosen so as toobtain a characteristic frequency f_(c) that satisfies the following:0.3f_(c)≦f₀, where 0.3 f_(c) is the low limit of the operating range ofthis device.

Thus, on average, the interfluxon distance is less than the saturationvalue of the line. The phenomenon of repulsive interaction between theflux quanta (fluxons) results in a spatial redistribution of the fluxquanta (fluxons) along the line, about a mean interfluxon value, bysmoothing around a mean value, corresponding to the mean value of thetime interval between two pulses. At the output, the signal has aconsiderably reduced standard deviation of the time intervals betweenpulses. In this way, the short-term noise or phase noise of the inputsignal is reduced.

The characteristics of a Josephson transmission line are mainly theinductances, which depend on the length of the line and on technology,especially the mutual inductance L_(m), and on the characteristics ofthe junctions, namely the critical current I_(c) and the normalresistance R_(n). In order not to overly complicate the drawing in FIG.2, these well-known characteristics of the Josephson junctions are shownonly for the first junction JJ₁.

FIG. 3 gives a practical embodiment of a phase reduction deviceaccording to the invention with a superconducting circuit of theJosephson transmission line type, comprising a plurality of Josephsonjunctions, in a planar technology based on a thin film of a high-T_(c)superconductor on a bicrystal substrate.

Two substrates 1 and 2, typically SrTiO₃ substrates or else MgO or YSZsubstrates, the crystal axes of which have an angle difference in thesurface plane, are bonded together. A superconducting film 3, typicallya film of a material of the YBa₂Cu₃O_(n) form, where 6≦n≦7, is deposited(by epitaxy) on the surface plane of the bicrystal astride the bond lineof the bicrystal substrate, so that a grain boundary 4 grows right alongthe bond, beneath the superconducting film, equivalent to an electricalbarrier. The film is then etched into a ladder pattern, each rung of theladder corresponding to a Josephson junction.

In the example, the width w of a rung is around 5 microns, the length 1of a rung is around 20 microns and the space h between two rungs is ofthe same order (20 microns) . The film itself has a width of a fewmicrons, for a thickness of a few tenths of a micron (for example 0.3μm) . The substrate has a thickness of a few hundred microns, typically300 to 1000 μm.

A current source (not shown) delivers a bias current to each of theJosephson junctions, typically of the order of 100 microamperes for thetechnology taken as example. In the example, this bias current isapplied between two current bias electrodes C and C′ formed on a portion3′ of the superconducting film 3, this portion being shaped (by etching)so as to distribute this current right along the line, by means ofcurrent feed branches provided in pairs b₁, b′₁, . . . b₁₀₀, b′₁₀₀,arranged on either side of the ladder forming the series of junctions.In the example, the current feed branch b₁, and its complementary branchb′₁ on the ground line side current-bias the two junctions JJ₁ and JJ₂located on either side of these branches. For a line comprising twohundred Josephson junctions, the current source is designed to deliver abias current of the order of a few tens of milliamps, for example 20 mA,distributed along the line.

The input and output signal electrodes A, M, B, M′, typically made ofcopper or gold, are formed at each end of the film, and on either sideof the grain boundary 4.

For example, a Josephson transmission line comprising two hundredjunctions, with a length of about 2 millimeters, with a criticaljunction current I_(c) of 125 microamperes and a normal resistance R_(n)of 2 ohms defining a characteristic frequency f_(c), wheref_(c)=I_(c)R_(n)/φ₀=125×10⁻⁶×2/2.07×10⁻¹⁵ weber=116 gigahertz, isdefined in technology based on niobium superconducting films (0.1 μmthin films) with a high critical temperature below 30 kelvin and with a100 microamperes bias current (<I_(c)) for each junction. If a clocksignal of fundamental frequency f₀(≦f_(c)/3) of around 50 to 100gigahertz and having pulses that are very offset over time (short-termnoise) is applied as input to this line, it is possible to provide asoutput a signal S_(out) whose white noise/signal ratio is lowered by afactor of 10, i.e. of around −130 to −140 dB_(c) (instead of −115 to−120 dB_(c) at the input).

FIG. 4 a shows schematically the lattice structure of the fluxonsgenerated in such a line under the effect of a voltage pulse signalS_(in) applied as input.

If the line is represented as a channel 5, the voltage pulses of thesignal S_(in) are injected at one end of this channel, at a clockfrequency f₀. Fluxons flx₁, flx₂, . . . flx_(m) are generated in thechannel 5 and are spatially organized along a one-dimensional latticecorresponding to the direction of propagation of the fluxons in theline.

Because a transmission line is used, that is to say a line comprising alarge number of junctions so that the statistics of large numbers apply(as opposed to a superconducting logic circuit of the type comprisingonly a small number of junctions, such as a shift register), a spatialredistribution effect occurs by the smoothing of the interfluxondistance around a mean value d₀, which corresponds to a mean value ofthe time interval between two pulses of the input signal. In otherwords, the standard deviation of the values of the time intervalsbetween the pulses in the output signal is reduced. More precisely, andshown in FIG. 4 b, the phase noise of the signal S_(in) applied as inputis manifested in this signal by a dispersed temporal distribution. Thefluxons generated by this signal are also spatially dispersed in theline, as shown schematically in FIG. 4 b. Since the characteristics ofthe line (f_(c)) are chosen so that the distance between the fluxonsgenerated by the input signal S_(in) is on average smaller than thesaturation value of the line, there is repulsive interaction between theclosest neighbor fluxons. In the figure, these repulsions are indicatedby arrows. In the example shown in this figure, it is assumed that thesaturation value corresponds to a time difference of 22 picoseconds.Thus, whenever the interfluxon distance corresponds to a time differencesmaller than this value, the mutual repulsion produces its (flx₁−flx₂,flx₂−flx₃, flx₄−flx₅) effects. If this distance is greater, there are no(flx₃−flx₄) effects. After a transient phase corresponding in practiceto around twenty pulses, the fluxons are spatially redistributed in theline around a smoothed value of the interfluxon distance. In the exampleshown schematically in FIG. 4 c, this smoothed value corresponds to atime difference between two pulses of the output signal S_(out) of 20picoseconds.

The output signal thus has its voltage pulses more uniformlydistributed, corresponding to a reduction in the phase noise level,compared with the signal level at the fundamental frequency f₀. Inpractice, with a transmission line as shown in FIG. 3, a reduction by afactor of 10 in the N₂/N₁ ratio (FIG. 1) may be observed.

The spatial separation and, therefore, the interactions depend on theratio of the fluxon propagation speed to the signal frequency. Thefluxon speed may be varied by modifying the bias current. The biascurrent may therefore be adjusted according to the frequency of theinput signal, if so required.

FIGS. 5 a and 5 b illustrate an alternative embodiment of a phasereduction device based on a superconducting Josephson transmission linecircuit. In this embodiment, the superconducting circuit comprises twoJosephson transmission lines. A substrate 1 and a substrate 1′ are thenbonded on either side of a substrate 2, to form a tricrystal substrate.A superconducting film is deposited on the zones 3 a and 3 b, one aboveeach bond line, so as to grow a respective grain boundary 4 a, 4 b. Inthese figures, the current feed branches distributed along the line arewires, typically copper wires, corresponding contact pads 6 beingprovided on the films.

Such a construction allows the effectiveness of the spatialredistribution in the lines to be improved, by adding another dimensionto the phenomenon of interaction between the fluxons. By placing thefilms on the zones 3 a and 3 b spaced apart with a gap such that thedistance between a fluxon in one film and a fluxon in the other film isshorter than the saturation value, the same interaction phenomenon isobserved. In other words, for a superconducting circuit based on twoJosephson transmission lines, the fluxons generated in the circuit areorganized along a two-dimensional periodic lattice. Typically, for thenumerical examples of the line characteristic and frequency (f₀) valuesgiven above, a gap of a few microns must be provided.

In order for this effect to be effective, it is necessary to favor astable (staggered) configuration of the two-dimensional periodic latticeof the fluxons with respect to the superconducting circuit, typically ona triangular base. Otherwise, the repulsion may have a random effect,being in the direction of propagation x of the line or in the oppositedirection. This is therefore an unstable situation. Referring to FIG. 5a, in which the two films forming the Josephson transmission lines areperfectly aligned along x and y, the desired lattice is obtained byphase-shifting the signal applied as input to the second line by π. Atwo-dimensional triangular-based periodic lattice is obtained, asillustrated in FIG. 5 b. The fluxon fix of a line then undergoes theinteractions due to four fluxons, namely two fluxons flx₁ and flx₂ oneither side of this fluxon flx, on the same line, and two fluxons flx₃and flx₄ on the other line, located on either side of the bisector 7 ofthis line passing through the fluxon flx.

The π phase shift may be applied in various ways, as shown in FIGS. 6 aand 6 b.

In FIG. 6 a, the π phase shift is applied to the input signal S_(in). Itis then preferable for the signal coming from the quasiperiodic source100 to be applied to a circuit 101 in order to be split into two asoutput. An example of this splitter circuit 101 produced in RSFQ logicis shown in detail in the figure, as a practical example. It deliverstwo signals in phase as output.

In FIG. 6 b, the π phase shift is applied to the output signal S_(out,1)of the first line, this signal being injected into the second line. Inthis case, the fluxons at the start of the first line benefit from thespatial redistribution already obtained at the output of this firstline—this is a positive feedback effect. An interconnection line 102 isthen provided in order to feed the output signal from the first line asinput for the phase shifter of the second line. This line is typicallyproduced in technology of the coplanar, strip or microstrip type, withmaterials that are compatible with the Josephson transmission linetechnology used, or may also be a Josephson transmission line.

The two Josephson transmission lines may not be accurately aligned onthe substrate, and the interconnection line 102 may also introduce adelay, such that the output signals S_(out,1) and S_(out,2) are notperfectly π phase-shifted. In this case, the interactions between thelines may not be optimal. Advantageously, the bias current I_(b) of oneor more junctions may advantageously be locally modified in order tolocally adapt the fluxon propagation speed. This correction is easilyapplied owing to the distribution of the current right along the line,by current feed branches (FIG. 3) or current feed wires (FIG. 5 a).Thus, provision is made for the bias current I_(b) of the junctions tobe preferably variable, this being able to be adjusted for each junctionor each group of junctions.

It is also possible to provide more than two transmission lines inparallel in the surface plane of the substrate. FIG. 6 c illustrates anexample of a circuit comprising three Josephson transmission lines. Toobtain a positive inter-line interaction effect, which favors thedisplacement of the fluxons along the propagation direction x of thelines, a central line Li₁, which receives the input signal S_(in) asinput, and two lines Li₂ and Li₃ on either side of it, which receive aπ-phase-shifted signal as input, which may be the input signal S_(in) asshown (in FIG. 6 a) or the output signal S_(out,1) of the first line(FIG. 6 b), are provided. Again, the fluxons are organized along atwo-dimensional periodic lattice, but the number of lines of thislattice is increased. In this way, the fluxons of the central line Li₁are subjected to the interactions from their own line and to theinteractions due to the other two lines, that is to say for each fluxonup to six interactions due to the six closest neighbor fluxons, two perline.

By increasing the number of lines in parallel, the number ofinteractions is increased. In the three-line example (FIG. 6 c), thecentral line Li₁ benefits from the interactions due to the two lines Li₂and Li₃ located on either side of it, but the lines Li₂ and Li₃ eachbenefit only from the interactions due to the line Li₁.

The choice of a larger number of lines will depend on the design of thedevice that the application can accept. It should be noted that in thecase of n lines in parallel, each line may then be made shorter, that isto say with fewer junctions, owing to the retroactive effect of theredistribution combined with the additional dimension of theinteractions in the two-dimensional lattice thus formed. The designs areevaluated in such a way that the statistics of large numbers can apply,in order to produce the desired effect of smoothing the interfluxondistances.

In general, in the case of n lines in parallel, signals are appliedalternately, namely the input signal to one line and then thephase-shifted input signal to the next line (by means of a phase shiftercircuit—FIG. 6 a). For example, the even-order lines receive the inputsignal (S_(in)) and the odd-order lines receive the phase-shifted inputsignal. The output signal of the device is delivered as output from oneof the lines.

FIG. 7 shows an example of a phase noise reduction device used in afrequency doubler circuit. In the example, the circuit comprises twolines in parallel, the first receiving the input signal S_(in) and theother the phase-shifted input signal. The first line delivers the signalS_(out,1) as output while the other line delivers the signal S_(out,2)as output.

The two lines are placed in such a way that the fluxons in the linesinteract with one another, reducing the short-term phase noise. The twooutput signals S_(out,1) and S_(out,2) thus obtained as output areapplied as inputs to an RSFQ (combiner) logic circuit, which delivers asoutput a signal S_((2f0)) having a frequency twice that of the inputsignal S_(in), with a low phase noise.

Thus, a phase noise reduction device according to the invention mayadvantageously be used in a frequency doubler circuit and more generallyin a frequency multiplier circuit, by circuit cascading of this type,while still maintaining an extremely low phase noise background.

FIG. 8 a shows another example of an embodiment of a Josephsontransmission line, which can be used in all the alternative embodimentsof a phase reduction device according to the invention that have justbeen described. FIG. 8 b may be used in a structure consisting of asingle line or of multiple lines, the lines then being stackedvertically. In these two FIGS. 8 a and 8 b, the lines are produced in aramp-edge junction technology, which is an SNS (standing forSuperconductor/Normal or insulating material/-Superconductor) multilayertechnology. The normal or insulating material is for example PrBaCuO,which is a nonsuperconductor, the material having a structure similar toYBaCuO, compatible with the lattice cell characteristics of thesuperconductor. A comb shape comprises a first superconducting film 9 (athin film) deposited on a heterostructure (8) of normal or insulatingmaterial deposited on the superconducting base electrode shown in grayin the figures, on a substrate. The teeth of the comb have the shape ofa ramp decreasing toward the substrate. A thin layer of insulation and asecond superconducting film 10 in the form of a comb are deposited onthe substrate, the end of the teeth of this comb being above the end ofthe teeth of the superconducting film 9 of the first comb. The junctionsJJ₁, JJ₂, . . . , etc. are thus formed in the plane at the point wherethe layer 8 of normal or insulating material is thinned, between the twosuperconductor films 9 and 10.

FIG. 8 b is a variant of FIG. 8 a in which the second superconductorfilm 10 is “folded” over the first film 9, which makes it possible tosignificantly save surface area.

FIG. 9 a shows another embodiment of a phase noise reduction deviceconsisting of a superconducting circuit based on a voltage pulsetransmission line. In this embodiment, the transmission line is producedby a long Josephson junction. Such a junction is typically obtained inan SIS trilayer technology, preferably based on the low-T_(c)superconductor: a thin film 20 of normal (or insulating) material (forexample Al₂O₃), forming a barrier between two layers 21 and 22 ofsuperconductor (for example niobium). A bias current i smaller than thecritical current I_(c) of the long Josephson junction is applied betweenthe two superconductor layers 21 and 22. Applying pulses to the input ofthe line generates vortex (Josephson vortex) fluxes in the layer ofnormal material which, under the effect of the bias (DC) current of theline (the Lorentz force), propagate toward the output. The flux quantumassociated with each vortex is equal to φ₀. The same repulsiveinteraction effects apply to these vortex fluxes generated under theeffect of the clock signal S_(in), which are organized in the line as aone-dimensional periodic lattice and which propagate along thepropagation direction x of the line.

In a typical embodiment, such a line will have a length of around onehundred nanometers.

Several of these lines may be placed in parallel in order to obtain thesame advantageous effects seen previously, by stacking them verticallyas shown in FIG. 9 b, this being feasible but more tricky.

The current is preferably distributed along the line as shown in FIG. 9b.

The level of the bias current may be adjusted according to the frequencyof the input signal.

Another embodiment of a phase noise reduction device according to theinvention is shown in figures 10 a and 10 b, which corresponds to a typeII superconductor circuit based on an active Abrikosov vortex flux-flowtransmission line. The Abrikosov vortex flux principle is briefly thefollowing: in the presence of an increasing magnetic field, thesuperconductor switches to a normal/superconductor hybrid state.Currents are generated in the surface of the superconductor which tendto shield the magnetic field. The magnetic flux that enters thesuperconductor is in the form of field lines grouped together on thesurface of a disk a few tens of ångstroms in radius. The flux containedin this small zone bounded by magnetic field shielding currents thatcirculate around it is equal to a flux quantum φ₀. These vortex fluxesare organized on the surface as a triangular-based periodic lattice, asshown in figure 11. By injecting a suitably directed DC current, thisvortex flux lattice propagates translationally, along a directionorthogonal to the current (Lorentz force).

One advantage of such a transmission line is that the vortex fluxes areorganized “naturally” as a triangular-based two-dimensional periodiclattice.

By suitably current-biasing the device, the application of anelectromagnetic signal as input generates a vortex flux lattice, whichmoves in lines L_(v) (FIG. 11) along this lattice structure. At theoutput, a receive device (any matched load) receives the associatedvoltage pulses.

Furthermore, if in the superconducting material used, for exampleNdBa₂Cu₃O₇, the twin planes are arranged in parallel, this organizationbecomes natural—the lines L_(v) correspond to the twin planes.

According to the invention, the active superconductor circuit comprises(FIGS. 10 a, 10 b), a film (thin layer) 13 of type II superconductor,such as YBa₂Cu₃O₇ or NdBa₂Cu₃O₇ deposited (by epitaxy) on a substrate12, for example an SrTiO₃ substrate. A slot 14 is made over the entirewidth of the film, leaving only a microbridge 15 of superconducting filmbetween the two parts 13 a and 13 b of the film, on either side of theslot. This microbridge has a height equal to the thickness of the filmor less. In the example, this microbridge has a height e of around 0.1microns, for a microbridge length L, along the direction of the slot,less than one hundred microns and a width W, which is also the width ofthe slot, of greater than one hundred microns.

Two bias electrodes 16 and 17, for applying a DC current i of about afew milliamperes, are provided at each end of the film. Two input signalelectrodes 18 and 19 are provided at one end of the slot, on each part13 a, 13 b of the film on either side of the slot, in order to apply theAC input signal S_(in), such that it imposes, periodically at the inputof the microbridge, a local magnetic field B_(e) which is greater thanthe critical field, so as to generate vortices v at the period of thissignal. The input signal may be a voltage pulse signal. It is alsopossible to apply an AC signal of the sinusoidal type. In practice, theclock signal source (not shown) is impedance-matched, relative to theimpedance of the microbridge (a few tens of ohms).

Two output signal electrodes 20 and 21 are provided at the other end ofthe slot, on each part 13 a, 13 b of the film on either side of theslot, in order to receive as input the voltage pulses corresponding tothe in-line transmission of the vortices (FIG. 11).

In practice, each voltage pulse (or each positive peak voltage of the ACsignal) passes through the local magnetic field B_(e) as input of themicrobridge above the critical field of the superconducting film causinga collection of vortices to nucleate. The DC current i appliedorthogonally (Lorentz force) along the appropriate direction causes thevortices to circulate.

The vortices are generated by modulating the magnetic field by the clocksignal applied as input. Suitable biasing of the circuit causes thevortices to propagate along the desired direction, toward the outputS_(out) of the device.

To further promote the displacement of the vortices in the desireddirection, it is possible to place the device in a low DC magnetic fieldB, for example of about twenty millitesla, suitably oriented so that thevortices are oriented in the same direction, for example by placing apair of Helmholtz coils on either side of the circuit.

Such a superconducting circuit may advantageously be used in a frequencydoubler stage as indicated above, with another similar circuitassociated with a phase shifter circuit, in a frequency multiplicationdevice.

Thus, in this embodiment, the transmission line comprises a film oftype-II superconductor in the hybrid state, deposited on a crystallinesubstrate. The film is current-biased at its ends and includes a slot inthe width direction, except at the place of a microbridge, the slotseparating the film into two parts. The quasiperiodic signal is appliedat one end of the slot, between the two parts of the film, and theoutput signal is obtained at the other end of the slot, between the twoparts of the film.

Advantageously, such a superconductor device is immersed in a DCmagnetic field oriented perpendicular to the surface plane of the slot.

The invention that has just been described thus uses the periodicstructure of the lattice of flux quanta (fluxons, vortices) that aregenerated and the repulsive interaction property of these flux quanta(which can be likened to magnetic dipoles) in order to reduce the phasenoise of a signal coming from a quasiperiodic source. This deviceaccording to the invention is advantageously used to deliver a multiplefrequency signal without a phase noise degradation.

The invention applies more particularly in the high-frequency and veryhigh-frequency field in rapid electronic systems. In particular, such adevice may be used in RSFQ logic circuits.

1. A device for reducing the phase noise of a quasiperiodic signalcoming from a quasiperiodic source of fundamental frequency f₀,comprising a physical system for transmitting pulses by transferringparticles, said physical system being defined so as to have acharacteristic frequency f_(c) defining an operating frequency range ofthe device with a low limit that is dependent on said characteristicfrequency, in such a way that, for the quasiperiodic signal applied asinput, said particles have a mutual repulsive interaction and saidphysical system delivering, as output, pulses at the fundamentalfrequency f₀.
 2. The device for reducing the phase noise of aquasiperiodic signal, coming from a quasiperiodic source of fundamentalfrequency f₀ as claimed in claim 1, comprising a superconducting circuitwith an active line for voltage pulse transmission by transferringquanta of flux φ₀, said circuit being defined so as to have acharacteristic frequency f_(c) such that 0.3 f_(c), is less than orequal to the fundamental frequency f₀ of the quasiperiodic signalapplied as input, and delivering, as output, a voltage pulse signal offundamental frequency f₀.
 3. The phase noise reduction device as claimedin claim 1, comprising at least two superconducting circuits, namely acircuit for a π phase shift of the input or of the output of one of saidcircuits and a combiner circuit for producing a frequency-doubling stagein a frequency multiplication circuit.
 4. The phase noise reductiondevice as claimed in claim 2, wherein the superconducting circuitcomprises a Josephson transmission line geometrically defined with saidcharacteristic frequency.
 5. The phase noise reduction device as claimedin claim 4, wherein the Josephson transmission line is a long Josephsonjunction.
 6. The phase noise reduction device as claimed in claim 4,wherein said transmission line comprises a plurality of parallel-shuntedJosephson junctions.
 7. The phase noise reduction device as claimed inclaim 6, wherein each Josephson transmission line is of the typecomprising a line with bicrystal junctions.
 8. The phase noise reductiondevice as claimed in claim 6, wherein each Josephson transmission lineis of the type comprising a line with ramp-edge junctions.
 9. The phasenoise reduction device as claimed in claim 5, wherein thesuperconducting circuit comprises several Josephson transmission linesplaced in parallel.
 10. The phase noise reduction device as claimed inclaim 9, wherein it comprises a π phase shift circuit at the input of atleast one transmission line, applying a phase-shifted signal to saidline.
 11. The phase noise reduction device as claimed in claim 10,wherein said phase shift circuit receives as input the input signal ofthe device.
 12. The phase noise reduction device as claimed in claim 10,wherein said phase shift circuit receives as input the output signalfrom a line.
 13. The phase noise reduction device as claimed in claim11, wherein the superconducting circuit comprises n Josephsontransmission lines of rank 1 to n in one and the same surface plane of asubstrate, with n an integer ≧2, and in that one signal among the inputsignal and the phase-shifted input signal is applied to the lines ofeven rank and the other signal is applied to the lines of odd rank, theoutput signal being delivered as output of one of the n lines.
 14. Thephase noise reduction device as claimed in claim 5, comprising currentbias means with a plurality of branches for feeding the current, inorder to distribute this current along each Josephson transmission line.15. The phase noise reduction device as claimed in claim 14, comprisingmeans for adjusting the intensity of the bias current according to thefrequency of the input signal.
 16. The phase noise reduction device asclaimed in claim 2, wherein the superconducting circuit comprises avortex flux-flow voltage-pulse transmission line.
 17. The phase noisereduction device as claimed in claim 16, wherein said transmission linecomprises a superconducting film of type II in the hybrid state,deposited on a crystalline substrate, said film being current-biased atits ends and comprising a slot in the width direction, except at thepoint of a microbridge, said slot separating the film into two parts,and wherein the quasiperiodic signal is applied to one end of the slot,between the two parts of the film, and the output signal is obtained atthe other end of the slot, between the two parts of the film.
 18. Thephase noise reduction device as claimed in claim 16, wherein saidsuperconducting device is immersed in a DC magnetic field orientedperpendicular to the surface plane of the slot.
 19. Phase noisereduction device as claimed in claim 1, wherein the superconductingcircuit or circuits use a high critical temperature superconductor. 20.Phase noise reduction device as claimed in claim 2, wherein thesuperconducting circuit or circuits use a high critical temperaturesuperconductor.